Sahih al-Bukhari 4000
In-book no 4000وَعَنْ مَجْزَأَةَ، عَنْ رَجُلٍ، مِنْهُمْ مِنْ أَصْحَابِ الشَّجَرَةِ اسْمُهُ أُهْبَانُ بْنُ أَوْسٍ وَكَانَ اشْتَكَى رُكْبَتَهُ، وَكَانَ إِذَا سَجَدَ جَعَلَ تَحْتَ رُكْبَتِهِ وِسَادَةً.
The same narration was told by Majzaa from a man called Uhban bin Aus who was one of those who had witnessed (the Pledge of allegiance beneath) the Tree., and who had some trouble in his knee so that while doing prostrations, he used to put a pillow underneath his knee.